The overall objective of the proposed studies is to determine the effects of intercostal muscle proprioceptors on the control of breathing. The first objective is to determine the effects of external and internal intercostal muscle spindle endings and tendon organs on medullary respiratory activity. Muscle spindle endings will be selectively stimulated by muscle vibration, and tendon organs will be selectively stimulated by ventral root stimulated muscle contraction. The second objective is to determine the medullary interneurons transmitting intercostal muscle proprioceptor activity to medullary bulbospinal respiratory neurons. The third objective is to determine which proprioceptors (primary and/or secondary muscle spindle endings, tendon organs) in lower-thoracic external and internal intercostal muscles have a facilitatory effect on phrenic motor activity (spinal "intercostal-to-phrenic" reflex). Knowledge of the reflex effects of these specific muscle proprioceptors will contribute to our understanding of the mechanisms by which respiratory muscle activity is altered undered various circumstances, such as changing posture, chest compression and expansion, pregnancy, obesity, respiratory muscle fatique, water immersion, and changes in chest wall mechanics resulting from lung diseases.